Pressure and temperature sensors using surface acoustic wave (SAW) devices are known in the art. It is well known that the velocity of a SAW is sensitive to temperature and stress or strain on its SAW substrate. It is also well known that external forces on the SAW substrate can generate strain fields at the surface which will perturb the SAW velocity. By way of example, this effect has been used to make SAW pressure sensors where the change in SAW velocity changes the frequency of resonators or where the change in SAW velocity changes the time delay of a reflected signal. SAW sensors offer advantages such as passive device operation (no battery), wireless operation, small size, low cost, rugged construction, and ease of production in high volume using standard process equipment, as described by way of example in U.S. Pat. No. 6,571,638, the disclosure of which is herein incorporated by reference. Physical parameters measured using SAW sensors include temperature, pressure, strain, acceleration, and torque.
U.S. Pat. No. 6,003,378 to Scherr et al. discloses a pressure sensor using a SAW delay line formed on a pressure sensing membrane with the delay line extending over an expanding and compressing regions of the membrane. Scherr '378 teaches a SAW pressure sensor having reflectors disposed on only one side of an interdigital transducer. A wirelessly interrogatable pressure sensor using SAW elements include a reflective delay line with reflectors positioned on a pressure sensing membrane extending over both an expanding and a compressing region of the membrane. When subjected to a change in pressure, the reflectors located in regions of compression and expansion undergo shifts in acoustic wave velocity and hence in the phase angle of the reflected signal. The shifts in phase angle provide information on the pressure change that has occurred. Plate bending is used in this device, requiring a much larger device than would otherwise be needed in order to achieve the desired complementary stress distributions within the substrate.
U.S. Pat. No. 6,571,638 to Hines et al. discloses a pressure and temperature sensor that comprises a hermetically sealed insulating package and an elastic, piezoelectric substrate deformably supported within the package and perpendicular to a long axis of the SAW substrate. Three SAW resonators are fixed to a bottom of the substrate, two of which are positioned in a partially staggered, parallel relationship along the substrate for experiencing a different frequency shift responsive to a deformation of the substrate. A third resonator has a long axis nonparallel to the long axes of the two parallel resonators. The temperature coefficients the two parallel resonators are substantially equivalent with that of the third being different. This difference permits a temperature change to be sensed and transmitted. An electromagnetic signal is sent to the sensor from a remote location, which signal has a frequency resonant with the three resonators. An input electromagnetic signal is received at the remote location from the sensor. The input signal is indicative of the pressure and the temperature within the environment.